Answer is: four iron atoms <span>are reacting with every three molecules of oxygen. Balanced chemical reaction: 4Fe + 3O</span>₂ → 2Fe₂O₃. From chemical reaction: n(Fe) : n(O₂) = 4 : 3. Ratios of atoms of iron and molecules of oxygen is 4 : 3.. There is four atoms of iron and six atoms of oxygen on both side of reaction.